Oxidation of methanol on perovskite-type La2-xSrxNiO4 (0 ≤ x ≤ 1) film electrodes modified by dispersed nickel in 1 M KOH

Finely-dispersed nickel particles are electrodeposited on high surface-area perovskite-type La_2-xSr_xNiO₄ (0 ≤ x ≤ 1) electrodes for possible use in a direct methanol fuel cell (DMFC). The study is conducted by cyclic voltammetry, chronoamperometry, impedance spectroscopy and anodic Tafel polarization techniques. The results show that the apparent electrocatalytic activities of the modified oxide electrodes are much higher than those of unmodified electrodes under similar experimental conditions; the observed activity is the greatest with the modified La_1.5Sr_0.5NiO₄ electrode. At 0.550 V (vs. Hg|HgO) in 1 M KOH + 1 M CH₃OH at 25 °C, the latter electrode delivers a current density of over 200 mA cm⁻², whereas other electrodes of the series produce relatively low values (65–117 mA cm⁻²). To our knowledge, such high methanol oxidation current densities have not been reported on any other non-platinum electrode in alkaline solution. Further, the modified electrodes are not poisoned by methanol oxidation intermediates/products.     

Preparation of graphene nanosheet/carbon nanotube/polyaniline composite as electrode material for supercapacitors

Graphene nanosheet/carbon nanotube/polyaniline (GNS/CNT/PANI) composite is synthesized via in situ polymerization. GNS/CNT/PANI composite exhibits the specific capacitance of 1035 F g⁻¹ (1 mV s⁻¹) in 6 M of KOH, which is a little lower than GNS/PANI composite (1046 F g⁻¹), but much higher than pure PANI (115 F g⁻¹) and CNT/PANI composite (780 F g⁻¹). Though a small amount of CNTs (1 wt.%) is added into GNS, the cycle stability of GNS/CNT/PANI composite is greatly improved due to the maintenance of highly conductive path as well as mechanical strength of the electrode during doping/dedoping processes. After 1000 cycles, the capacitance decreases only 6% of initial capacitance compared to 52% and 67% for GNS/PANI and CNT/PANI composites.     

Carbon nanofibre/hydrous RuO2 nanocomposite electrodes for supercapacitors

Amorphous RuO₂·xH₂O and a VGCF/RuO₂·xH₂O nanocomposite (VGCF = vapour-grown carbon fibre) are prepared by thermal decomposition. The morphology of the materials is investigated by means of scanning electron microscopy. The electrochemical characteristics of the materials, such as specific capacitance and rate capability, are investigated by cyclic voltammetry over a voltage range of 0–1.0 V at various scan rates and with an electrolyte solution of 1.0 M H₂SO₄. The specific capacitance of RuO₂·xH₂O and VGCF/RuO₂·xH₂O nanocomposite electrodes at a scan rate of 10 mV s⁻¹ is 410 and 1017 F g⁻¹, respectively, and at 1000 mV s⁻¹ are 258 and 824 F g⁻¹, respectively. Measurements of ac impedance spectra are made on both the electrodes at various bias potentials to obtain a more detailed understanding of their electrochemical behaviour. Long-term cycle-life tests for 10⁴ cycles shows that the RuO₂·xH₂O and VGCF/RuO₂·xH₂O electrodes retain 90 and 97% capacity, respectively. These encouraging results warrant further development of these electrode materials towards practical application.     

Electrochemical properties of manganese oxide coated onto carbon nanotubes for energy-storage applications

Birnessite-type manganese dioxide (MnO₂) is coated uniformly on carbon nanotubes (CNTs) by employing a spontaneous direct redox reaction between the CNTs and permanganate ions (MnO₄⁻). The initial specific capacitance of the MnO₂/CNT nanocomposite in an organic electrolyte at a large current density of 1 A g⁻¹ is 250 F g⁻¹. This is equivalent to 139 mAh g⁻¹ based on the total weight of the electrode material that includes the electroactive material, conducting agent and binder. The specific capacitance of the MnO₂ in the MnO₂/CNT nanocomposite is as high as 580 F g⁻¹ (320 mAh g⁻¹), indicating excellent electrochemical utilization of the MnO₂. The addition of CNTs as a conducting agent improves the high-rate capability of the MnO₂/CNT nanocomposite considerably. The in situ X-ray absorption near-edge structure (XANES) shows improvement in the structural and electrochemical reversibility of the MnO₂/CNT nanocomposite after heat-treatment.     

Improved pseudocapacitive performance and cycle life of cobalt hydroxide on an electrochemically derived nano-porous Ni framework

The pseudocapacitance and morphology of an electrodeposited cobalt hydroxide (Co(OH)₂) significantly depends on the architecture of the electrode substrate. The nano-porous Ni framework, derived from the selective dissolution of Cu from a Ni-Cu alloy, effectively promotes the electrochemical utilization of deposited Co(OH)₂ even at a high loading amount condition. The great electronic and ionic conduction within the nano-structured electrode improves the energy storage performance of Co(OH)₂ as compared to that for a conventional flat Ni substrate. In this work, the Co(OH)₂ mass specific capacitance, evaluated using cyclic voltammetry (CV), only slightly decreases from 2650 to 2470 F g⁻¹ when the potential sweep rate is substantially increased from 5 to 200 mV s⁻¹. The developed Ni(OH)₂/NiOOH (from the nano-porous framework) incorporates with the deposited Co(OH)₂ upon CV cycling; the mixed hydroxide shows a noticeably synergistic capacitance. Furthermore, the dissolution of Co(OH)₂ in KOH electrolyte is greatly suppressed due to the incorporation of Ni(OH)₂/NiOOH, consequently prolonging the electrode cycle life.     

NiOx nanoparticles supported on polyethylenimine functionalized CNTs as efficient electrocatalysts for supercapacitor and oxygen evolution reaction

Ni oxide based nanoparticles (NPs) have been widely used as electrocatalysts in the electrochemical energy storage and conversion applications. In this paper, NiO_x NPs are successfully synthesized by the self-assembly of Ni precursor onto polyethylenimine functionalized carbon nanotubes (PEI-CNTs) assisted with microwave radiation. NiO_x NPs with size around 2–3 nm are homogenously dispersed on the PEI-CNTs supports with no aggregation. The electrochemical activity of NiO_x NPs on PEI-CNTs, NiO_x/PEI-CNTs, as effective electrocatalysts is studied for supercapacitor and oxygen evolution reaction in alkaline solutions. NiO_x/PEI-CNTs show a capacitance of 1728 and 1576 F g⁻¹ based on active material, and 221 and 394 F g⁻¹ based on total catalyst loading on 12.5% and 25% NiO_x/PEI-CNTs, respectively, which is substantially higher than 152 F g⁻¹ of unsupported NiO. The NiO_x/PEI-CNTs electrodes exhibit reversible and stale capacitance of ∼1200 F g⁻¹ based on active materials after 2000 cycles at a high current density of 10 A g⁻¹. NiO_x/PEI-CNTs also exhibit significantly higher activities for oxygen evolution reaction (OER) of water electrolysis, achieving a current density of 100 A g⁻¹ at an overpotential of 0.35 V for 25% NiO_x/PEI-CNTs. It is believed that the uniformly dispersed nano-sized NiO_x NPs and synergistic effect between the NiO_x NPs and PEI-CNTs is attributed to the high electrocatalytic performance of NiO_x/PEI-CNTs electrocatalysts. The results demonstrate that NiO_x NPs supported on PEI-CNTs are highly effective electrocatalysts for electrochemical energy storage and conversion applications.     

Capacitance studies of cobalt compound nanowires prepared via electrodeposition

The capacitor properties of cobalt compound nanowire (CCNW) electrodes, prepared by the one-step electroreduction of [Co(NH₃)₆]³⁺ in water, have been investigated. The CCNW electrode changes its various properties during its growth. During the initial growth stage, the CCNW electrodes consist of nanowires with smooth surfaces and have a specific capacitance (C_m) of 310 F g⁻¹. During the middle stage, prickles grow on the CCNW surface, leading to a reduction in its real surface area and its C_m value to 230 F g⁻¹. During the final stage, further growth of the prickles is accompanied by the fusion of the CCNWs, and hence, a drastic decrease in the real surface area. However, a maximum capacitance of C_m = 420 F g⁻¹ was obtained during this stage. This unexpected capacitance change was discussed in terms of the effects of rapid ion transfer and the electroactive material/electrolyte interface area. In addition, the aging effect and the cycle life of the CCNW electrode were also investigated.     

Preparation and electrochemical properties of Sr-doped Nd2NiO4 cathode materials for intermediate-temperature solid oxide fuel cells

Cathode materials Nd_2 − xSr_xNiO₄ were prepared by the glycine-nitrate process and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), AC impendence spectroscopy and DC polarization method, respectively. The results show that no reaction occurred between the electrode and the CGO electrolyte at 1100 °C and the electrode formed good contact with the electrolyte after being sintered at 1000 °C for 4 h. The rate-limiting step for oxygen reduction reaction on Nd_1.6Sr_0.4NiO₄ electrode changed with oxygen partial pressure and measurement temperature. The Nd_1.6Sr_0.4NiO₄ electrode gave a polarization resistance of 0.93 Ω cm² at 700 °C in air, which indicates that Nd_2 − xSr_xNiO₄ electrode is a promising cathode material for intermediate-temperature solid oxide fuel cell (IT-SOFC).     

Capacitance decay of nanoporous nickel hydroxide

Nanoporous nickel hydroxide Ni(OH)₂ coated on nickel foam by using a chemical bath deposition method shows a high specific capacitance of 2200 F g⁻¹ at a discharging current density of 1 Ag⁻¹. After 500 charge-discharge cycles, the specific capacitance is stabilized at 1470 Fg⁻¹, and there is only a 5% fall in specific capacitance during the following 1500 cycles. The relationship between the capacitance decay and changes in the microstructure and morphology of nanoporous Ni(OH)₂ is investigated. The results show that phase transformation and the growth of particle/crystal size, rather than the formerly proposed flaking off of Ni(OH)₂, are the major factors contributing to the capacitance decay.     

Synthesis and characterization of spherical nonstoichiometric Ni(OH)x (x = 2.03-2.10) as electrode materials

The spherical nonstoichiometric Ni(OH)_x (x = 2.03-2.10), as a new positive electrode material for Ni/MH batteries, are synthesized by spherical β-Ni(OH)₂ surface modified with chemically oxidized NiOOH nanoparticles. The average nickel oxidation state, microstructure and morphology of the spherical nonstoichiometric Ni(OH)_x are investigated by complexometric titration, X-ray diffraction (XRD), scamming electron microscopy (SEM) and transmission electron microscopy (TEM). It is demonstrated that the NiOOH with a flaky-like morphology are dispersed randomly on the surface of the spherical β-Ni(OH)₂. The effect of NiOOH on the electrochemical performance of spherical nonstoichiometric Ni(OH)_x is studied by galvanostatic charge-discharge experiments and cyclic voltammetry. Compared with the spherical β-Ni(OH)₂, the spherical nonstoichiometric Ni(OH)_x (x = 2.05) has an enhanced discharge capacity (300 mAh g⁻¹ at 0.2 C), higher discharge potential plateau and superior cycle stability. The existence of chemically oxidized NiOOH nanoparticles in the nickel electrode contributes great effect on the improvement of electrochemical performance.     

Cathodic deposition and characterization of tin oxide coatings on graphite for electrochemical supercapacitors

Amorphous tin oxide (SnO_x) was cathodically deposited onto graphite electrode in a bath containing 0.1 M stannous chloride (SnCl₂), 0.5 M sodium nitrate (NaNO₃), and 0.4 M nitric acid (HNO₃) in an aqueous solution of 50% (v/v) ethanol. The SnO_x coatings grown on graphite were characterized as typical capacitive behaviors by cyclic voltammetry (CV), chronopotentiometric (CP) in 0.5 M KCl. Specific capacitance (in milli-farad per square centimeter, C_a) changes linearly with the deposition charge up to 4.5 C cm⁻², and a maximum of as high as 355 mF cm⁻² was obtained with the SnO_x coating grown at around 5 C cm⁻². For the SnO_x coating deposited at 0.2 C cm⁻², a maximum specific capacitance (in farad per gram, C_m) of 298 and 125 F g⁻¹ was achieved from CVs at a scan rate of 10, and 200 mV s⁻¹, respectively. The value of C_m significantly gets lower from 265 to around 95 F g⁻¹ when the deposition charge increases from 0.2 to around 6.0 C cm⁻². The long cycle-life and stability of the SnO_x coatings on graphite via the presented cathodic deposition were also demonstrated.     

Characterization of honeycomb-like “β-Ni(OH)2” thin films synthesized by chemical bath deposition method and their supercapacitor application

Nanostructured nickel hydroxide thin films are synthesized via a simple chemical bath deposition (CBD) method using nickel nitrate Ni(NO₃)₂ as the starting material. The deposition process is based on the thermal decomposition of ammonia-complexed nickel ions at 333 K. The structural, surface morphological, optical, electrical and electrochemical properties of the films are examined. The nanocrystalline “β” phase of Ni(OH)₂ is confirmed by the X-ray diffraction analysis. Scanning electron microscopy reveals a macroporous and interconnected honeycomb-like morphology. Optical absorption studies show that “β-Ni(OH)₂” has a wide optical band-gap of 3.95 eV. The negative temperature coefficient of the electrical resistance of “β-Ni(OH)₂”, is attributed to the semiconducting nature of the material. The electrochemical properties of “β-Ni(OH)₂” in KOH electrolyte are examined by cyclic voltammetric (CV) measurements. The scan-rate dependent voltammograms demonstrate pseudocapacitive behaviour when “β-Ni(OH)₂” is employed as a working electrode in a three-electrode electrochemical cell containing 2 M KOH electrolyte with a platinum counter electrode and a saturated calomel reference electrodes. A specific capacitance of ∼398 × 10³ F kg⁻¹ is obtained.     

Adjustment of electrodes potential window in an asymmetric carbon/MnO2 supercapacitor

The electrodes mass ratio of MnO₂/activated carbon supercapacitors has been varied in order to monitor its influence on the potential window of both electrodes and consequently to optimize the operating voltage. It appeared that the theoretical mass ratio (R = 2), calculated considering an equivalent charge passed across both electrodes, is underestimated. It was demonstrated that R values of 2.5-3 are better adapted for this system; the extreme potential reached for each electrode is close to the stability limits of the electrolyte and active material, allowing a maximum voltage to be reached. During galvanostatic cycling up to 2 V, the best performance was obtained with R = 2.5. The specific capacitance increased from 100 to 113 F g⁻¹ during the first 2000 cycles, then decayed up to 6000 cycles and finally stabilized at 100 F g⁻¹. SEM images of the manganese based electrode after various numbers of thousands cycles exhibited dramatic morphological modifications. The later are suspected to be due to Mn(IV) oxidation and dissolution at high potential values. Hence, the evolution of specific capacitance during cycling of the asymmetric capacitor is ascribed to structural changes at the positive electrode.     

Preparation and electrochemical capacitance performances of super-hydrophilic conducting polyaniline

Super-hydrophilic conducting polyaniline was prepared by surface modification of polyaniline using tetraethyl orthosilicate in water/ethanol solution, whereas its conductivity was 4.16 S cm⁻¹ at 25 °C. And its electrochemical capacitance performances as an electrode material were evaluated by the cyclic voltammetry and galvanostatic charge/discharge test in 0.1 M H₂SO₄ aqueous solution. Its initial specific capacitance was 500 F g⁻¹ at a constant current density of 1.5 A g⁻¹, and the capacitance still reached about 400 F g⁻¹ after 5000 consecutive cycles. Moreover, its capacitance retention ratio was circa 70% with the growth of current densities from 1.5 to 20 A g⁻¹, indicating excellent rate capability. It would be a promising electrode material for aqueous redox supercapacitors.     

Application of activated carbon/DNA composite electrodes to aqueous electric double layer capacitors

A novel capacitor electrode auxiliary, deoxyribonucleic acid (DNA), is applied to an electric double layer capacitor (EDLC) containing an aqueous 3.5 M NaBr electrolyte. The present electrode is composed of activated carbon (95 wt.%) and DNA (2.5 wt.%) with polytetrafluoroethylene (PTFE) as a binder (2.5 wt.%). An EDLC cell with the DNA-loading electrodes exhibits improved rate capability and discharge capacitance. An EDLC cell with DNA-free electrodes cannot discharge above a current density of 3000 mA g⁻¹ (of the electrode), while a cell with the DNA-loading electrodes can work at least up to 6000 mA g⁻¹. Moreover, an open-circuit potential (OCP) of the DNA-loading electrode sifts negatively with ca. 0.2 V from an OCP of the corresponding electrode without DNA. It is noteworthy that a small amount of DNA loading (2.5 wt.%) to the activated carbon electrode not only improves the rate capability but also adjusts the working potential of the electrode to a more stable region.     

Preparation and electrochemical properties of a Sm2−xSrxNiO4 cathode for an IT-SOFC

Cathodic materials Sm_2−xSr_xNiO₄ (0.5 ≤ x ≤ 1.0) for an IT-SOFC (intermediate temperature solid oxide fuel cell) were prepared by the glycine-nitrate process and characterized by XRD, SEM, ac impedance spectroscopy and dc polarization measurements. The results showed that no reaction occurred between the Sm_2−xSr_xNiO₄ electrode and the Ce_0.9Gd_0.1O_1.9 (CGO) electrolyte at 1100 °C, and the electrode formed good contact with the electrolyte after sintering at 1000 °C for 2 h. The electrochemical properties of these cathode materials were studied using impedance spectroscopy at various temperatures and oxygen partial pressures. Sm_1.0Sr_1.0NiO₄ exhibited the lowest cathodic overpotential. The area specific resistance (ASR) was 3.06 Ω cm² at 700 °C in air.     

Binderless fabrication of amorphous RuO2 electrode for electrochemical capacitor using spark plasma sintering technique

The spark plasma sintering (SPS) technique was successfully used to mold a hydrous amorphous RuO₂electrode without any additives and binders. At the cyclic voltammetry (CV) scan rate of 1 mV s⁻¹, the electrochemical capacitances of the RuO₂ electrodes are 600-700 F g⁻¹ for the entire electrode. An increase in the SPS current during the compaction led to the crystallization and dehydration of RuO₂, which in turn, resulted in a significant decrease in its capacitance. There is room to improve the rate properties as we observed a steep drop in the capacitance when the CV scan rate was raised.     

Study and optimisation of manganese oxide-based electrodes for electrochemical supercapacitors

A manganese oxide material was synthesised by an easy precipitation method based on reduction of potassium permanganate(VII) with a manganese(II) salt. The material was treated at different temperatures to study the effect of thermal treatment on capacitive property. The best capacitive performance was obtained with the material treated at 200 °C. This material was used to prepare electrodes with different amounts of polymer binder, carbon black and graphite fibres to individuate the optimal composition that gave the best electrochemical performances. It was found that graphite fibres improve the electrochemical performance of electrodes. The highest specific capacitance (267 F g⁻¹ MnO_x) was obtained with an electrode containing 70% of MnO_x, 15% of carbon black, 10% of graphite fibres and 5% of PVDF. This electrode, with CB/GF ratio of 1.5, showed a higher utilization of manganese oxide. The results reported in the present paper further confirmed that manganese oxide is a very interesting material for supercapacitor application.     

Potentiostatically deposited nanostructured CoxNi1−x layered double hydroxides as electrode materials for redox-supercapacitors

Cobalt–nickel layered double hydroxides (Co_xNi_1−x LDHs) were deposited onto stainless steel electrodes by the potentiostatic deposition method at −1.0 V vs. Ag/AgCl using various molar ratios of Co(NO₃)₂ and Ni(NO₃)₂ in distilled water. Their structure and surface morphology were studied by using X-ray diffraction analysis, energy dispersive X-ray spectroscopy and scanning electron microscopy. A network of Co_xNi_1−x LDH nanosheets was obtained. The nature of the cyclic voltammetry and charge–discharge curves suggested that the Co_xNi_1−x LDHs exist in the form of solid solutions. The capacitive characteristics of the Co_xNi_1−x LDHs in 1 M KOH electrolyte showed that Co_0.72Ni_0.28 LDHs had the highest specific capacitance value, 2104 F g⁻¹, which is also the highest yet reported value for oxide materials in general.     

Solid state synthesis of hydrous ruthenium oxide for supercapacitors

A novel solid state route has been successfully developed for the synthesis of nano-scale hydrous ruthenium oxide (denoted as RuO₂·xH₂O). The procedure involves directly mixing RuCl₂·xH₂O with alkali to form RuO₂·xH₂O in a mortar at room temperature. Transmission electron microscopy (TEM) and N₂ adsorption–desorption measurement indicate that the RuO₂·xH₂O particle is approximately 30–40 nm with mesoporous structure. The crystalline structure and the electrochemical properties of RuO₂·xH₂O have been systematically explored as a function of annealing temperature. At lower temperatures, the RuO₂·xH₂O powder was found in an amorphous phase and the maximum capacitance of 655 F g⁻¹ was obtained by annealing at 150 °C. Higher temperatures (exceeding 175 °C) presumably converted amorphous phase into crystalline one and the corresponding specific capacitance dropped rapidly from 547 F g⁻¹ at 175 °C to 87 F g⁻¹ at 400 °C. Also, the dependence of electrochemical performance on annealing conditions of RuO₂·xH₂O was investigated by electrical impedance spectroscopy (EIS) study.