Development of NiMH-based Fuel Cell/Battery (FCB) system: Characterization of Ni(OH)2/MnO2 positive electrode for FCB

The performance of the positive electrode composed of a mixture of nickel hydroxide (Ni(OH)₂) and a small amount of manganese dioxide (MnO₂) was investigated for the positive electrode of Fuel Cell/Battery (FCB) system. It was found that the positive electrode can function not only as an active material of secondary batteries when it is charged but also as a catalyst of fuel cells when oxygen is supplied, which was confirmed by the following characterization: electrochemical characterization was performed with cyclic voltammetry (CV) and galvanostatic discharge curve in oxygen and oxygen-free atmosphere. CV of Ni(OH)₂/MnO₂ positive electrode exhibited the redox reaction of Ni(OH)₂ as well as oxygen reduction reaction. It was observed that the discharge curves of positive electrode had two working potentials in half cell test when the electrode was charged and oxygen was supplied: one from the reactions of nickel oxyhydroxide (NiOOH); the other from the fuel cell reactions of manganese dioxide (MnO₂). It was also observed that the discharge curves had two working voltages in full cell test when the cell was fully charged and oxygen was supplied: one at 1.2 V from the battery reactions of NiOOH; the other at 0.8 V from the fuel cell reactions of MnO₂. In particular, the discharge capacity of overcharged cell was improved approximately 2 times compared with a battery of the same electrode quantity due to the additional function of this system as a fuel cell by using oxygen generated by water electrolysis. XRD analysis showed that there was no crystal structure change before and after (over)charge–discharge cycles. In summary, these experimental results showed that the novel bi-functional FCB system could provide an improved overall energy density per weight compared with conventional secondary batteries.     

Rapid hydrogen charging on metal hydride negative electrode of Fuel Cell/Battery (FCB) systems

The characteristics of rapid gaseous H₂ charging/electrochemical discharging of the metal hydride negative electrode were investigated for the application in Fuel Cell/Battery (FCB) systems. They were evaluated with the H₂ gas absorption, followed by the subsequent electrochemical discharging in the electrolyte solution (6M KOH). Then, the cyclability of charge–discharge was also examined. It was observed that more than 70% of the theoretical capacity was charged within 10 min with 0.3 MPa and 0.5 MPa of the initial H₂ pressures. The electrochemical discharge curve showed that more than 86% of the absorbed H₂ was discharged. Furthermore, the cycled charge–discharge process indicated that the H₂ gas charge and electrochemical discharge process is an effective way to rapidly charge and activate the metal hydride without degeneration.     

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.     

Lithium insertion into manganese dioxide electrode in MnO2/Zn aqueous battery: Part I. A preliminary study

The discharge characteristics of manganese dioxide (γ-MnO₂ of electrolytic manganese dioxide (EMD) type) as a cathode material in a Zn–MnO₂ battery containing saturated aqueous LiOH electrolyte have been investigated. The X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) data on the discharged material indicate that lithium is intercalated into the host structure of EMD without the destruction of its core structure. The XPS data show that a layer of insoluble material, possibly Li₂CO₃, is deposited on the cathode, creating a barrier to H₂O, thus preventing the formation of Mn hydroxides, but allowing the migration of Li ions into the MnO₂ structure. The cell could be reversibly charged with 83% of voltaic efficiency at 0.5 mA/cm² current density to a 1.9 V cutoff voltage. The percentage utilization of the cathode material during discharge was 56%.     

Poly(3-methylthiophene)/MnO2 composite electrodes as electrochemical capacitors

Composite electrodes prepared by electrodeposition of manganese oxide on titanium substrates modified with poly(3-methylthiophene) (PMeT) were investigated and compared with Ti/MnO₂ electrodes. The polymer films were prepared by galvanostatic deposition at 2 mA cm⁻² with different deposition charges (250 and 1500 mC cm⁻²). The electrodes were characterized by cyclic voltammetry in 1 mol L⁻¹ Na₂SO₄ and by scanning electron microscopy. The results show a very significant improvement in the specific capacitance of the oxide due the presence of the polymer coating. For Ti/MnO₂ the specific capacitance was of 122 F g⁻¹, while Ti/PMeT₂₅₀/MnO₂ and Ti/PMeT₁₅₀₀/MnO₂ displayed values of 218 and 66 F g⁻¹, respectively. If only oxide mass is considered, the capacitances of the composite electrode increases to 381 and 153 F g⁻¹, respectively. The micrographs of samples show that the polymer coating leads to very significant changes in the morphology of the oxide deposit, which in consequence, generate the improvement observed in the charge storage property.     

MnO2 nanotube and nanowire arrays by electrochemical deposition for supercapacitors

Highly ordered MnO₂ nanotube and nanowire arrays are successfully synthesized via a electrochemical deposition technique using porous alumina templates. The morphologies and microstructures of the MnO₂ nanotube and nanowire arrays are investigated by field emission scanning electron microscopy and transmission electron microscopy. Electrochemical characterization demonstrates that the MnO₂ nanotube array electrode has superior capacitive behaviour to that of the MnO₂ nanowire array electrode. In addition to high specific capacitance, the MnO₂ nanotube array electrode also exhibits good rate capability and good cycling stability, which makes it promising candidate for supercapacitors.     

Application of a Cu-CeO2/Ni-yttria-stabilized zirconia multi-layer anode for anode-supported Solid Oxide Fuel Cells operating on H2-CO syngas fuels

A Ni-yttria-stabilized zirconia (YSZ) anode and a Cu-CeO₂/Ni-YSZ multi-layer anode have been fabricated for use in anode-supported Solid Oxide Fuel Cells (SOFCs), and their performances and stabilities in H₂-CO syngas have been studied at 750 °C. A high CO content has been found to cause carbon deposition and crack formation in the Ni-YSZ anode after long-term operation, but the Cu-CeO₂ catalyst layer on the Ni-YSZ anode surface improves its stability in syngas with high CO content by facilitating the water gas shift reaction. The optimized single cell has run in syngas with a composition of 48.5%H₂-48.5%CO-3%H₂O for 460 h without obvious degradation of its performance, however, its performance decreases after 630 h due to carbon deposition in the anode functional layer and subsequent crack formation on the anode and electrolyte.     

Hierarchically structured TiO2@MnO2 nanowall arrays as potential electrode material for high-performance supercapacitors

We have reported a facile route for the fabrication of TiO₂@MnO₂ core–shell nanostructures for use as an electrode material, using a simple hydrothermal process for supercapacitor applications. Field-emission scanning electron microscopy and transmission electron microscopy studies confirmed the formation of a MnO₂ nanowall shell structure on the core of TiO₂ nanorod surfaces. The nanostructured TiO₂@MnO₂ core–shell was used as an electrode material, which exhibited excellent electrochemical properties with an improved areal capacitance of 22.19 mF cm⁻² (TM-3) at a scan rate of 5 mV s⁻¹ in a 1-M Na₂SO₄ electrolyte solution. Moreover, the electrode material demonstrated excellent performance with long term cycling stability, by retaining 85% of its initial capacitance after 4000 cycles.     

Electrochemical investigation of MnO2 electrode material for supercapacitors

MnO₂ electrode material is synthesized by low temperature solid state reaction between KMnO₄ and MnCl₂. Effects of the KMnO₄:MnCl₂ molar ratio on the structure, morphology and electrochemical properties of the as-prepared sample were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical tests. Results showed that the obtained MnO₂ is α-MnO₂, the average diameter is about 0.5-1.5 μm, which are constituted of nanoparticles of 20 nm. Under 100 mA g⁻¹, the specific capacitances of the prepared sample is 258.7, 219.6, 215.3, 198.5 and 209.5 F g⁻¹ at the KMnO₄/MnCl₂ molar ratio of 3:2, 2:1, 1:1, 1:2 and 2:3, respectively. And the MnO₂ sample with a KMnO₄/MnCl₂ molar ratio of 3:2 exhibits the best discharge capacitance and cycle performance. When the charge/discharge rate increases to 300 mA g⁻¹, the sample still remains initial discharge capacitance of 165.3 F g⁻¹, and the discharge capacitance is 145.9 F g⁻¹ after 200 cycles, the capacitance retention rate is 102.4% during the 20-200th cycles. Therefore, the MnO₂ sample is an excellent material for use in supercapacitors because of its large specific capacitance and good cycle performance.     

Anodic behavior of zinc in Zn-MnO2 battery using ERDA technique

The commercial, alkaline zinc-manganese dioxide (Zn-MnO₂) primary battery has been transformed into a secondary battery using lithium hydroxide electrolyte. Galvanostatic discharge–charge experiments showed that the capacity decline of the Zn-MnO₂ battery is not caused by the MnO₂ cathode, but by the zinc anode. The electrochemical data indicated that a rechargeable battery made of porous zinc anode can have a larger discharge capacity of 220 mAh/g than a planar zinc anode of 130 mAh/g. The cycling performance of these two anodes is demonstrated. Structural and depth profile analyses of the discharged anodes are examined by X-ray diffraction (XRD) and elastic recoil detection analysis (ERDA) techniques.     

Characterization of Pt supported on commercial fluorinated carbon as cathode catalysts for Polymer Electrolyte Membrane Fuel Cell

Pt nanoparticles supported on carbon monofluoride (CFx), synthesized from H₂PtCl₆ using NaHB₄ as a reducing agent has been investigated as a cathode electrocatalyst in fuel cells. Surface characterization, performed by transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD), shows a homogeneous distribution and high dispersion of metal particles. Kinetic parameters for the electrocatalyst are also obtained from the steady state measurements using a rotating disk electrode (RDE) in 0.5 M H₂SO₄ solution. Analysis by Koutecky–Levich equation indicates an overall 4 e^? oxygen reduction reaction (ORR). Evaluation of the catalyst in single cell membrane electrode assemblies (MEAs) for proton exchange membrane based Direct Methanol Fuel Cell (DMFC) and H₂ Fuel Cell at different temperatures and flows of O₂ and Air are shown and compared against commercial Pt/C as the cathode electrocatalyst. Evaluation of Pt/CFx in H₂ fed fuel cells shows a comparable performance against a commercial catalyst having a higher platinum loading. However, in direct methanol fuel cell cathodes, an improved performance is observed at low O₂ and air flows showing up to 60–70% increase in the peak power density at very low flows (60 mL min^?1).     

Experimental analysis of SO2 effects on Molten Carbonate Fuel Cells

The aim of this work is to investigate how SO₂ can affect MCFC performance and to discover the possible mechanisms involved in cathode sulphur poisoning, specifically considering the possible use of MCFCs in CCS (Carbon Capture and Storage) application. The different contributions of cathodic, anodic and electrolyte reactions have been considered to get a complete picture of the evolution of performance degradation. Experimental tests have been performed at the Fuel Cell Centre laboratories of Korea Institute of Science and Technology (KIST) thanks to 100 cm² single cell facilities and comparing results using both an optimized gas for laboratory conditions and a gas composition that simulates MCFCs when running in a Natural Gas Combined Cycle (NGCC) power plant. Polarisation curves, endurance tests, impedance measurements and gas analyses have been carried out to support the investigation.     

High performance composite interconnect La0.7Ca0.3CrO3/20 mol% ReO1.5 doped CeO2 (Re = Sm,Gd, Y) for solid oxide fuel cells

This paper reports and discusses composite interconnect materials that were modified from La_0.7Ca_0.3CrO_3−δ (LCC) by addition of Re doped CeO₂ (Re = Sm, Gd, Y) for improved conductivity at relative low temperatures. It is found that the addition of small amounts of RDC (ReO_1.5 doped CeO₂) into LCC dramatically increased the electrical conductivity. For the best system studied, LCC + 5 wt% SDC (Sm_0.2Ce_0.8O_1.9), LCC + 3 wt% GDC (Gd_0.2Ce_0.8O_1.9) and LCC + 3 wt% YDC (Y_0.2Ce_0.8O_1.9), the electrical conductivities reached 687.8, 124.6 and 104.8 S cm⁻¹ at 800 °C in air, respectively. The electrical conductivities of the specimens, LCC + 3 wt% SDC, LCC + 1 wt% GDC and LCC + 2 wt% YDC in H₂ at 800 °C were 7.1, 3.8 and 5.9 S cm⁻¹, respectively. With the increase of RDC content, the relative density increased, indicating that RDC served as an effective sintering aid in enhancing the sinterability of the powders. The average coefficient of thermal expansion (CTE) at 30–1000 °C in air increased with the increase of the RDC content. The oxygen permeation measurements indicated a negligible oxygen ionic conduction, indicating that the efficiency loss of a solid oxide fuel cell by permeation is negligible for the general cell design using LCC + RDC as interconnect. Therefore, the composite materials La_0.7Ca_0.3CrO₃/20 mol% ReO_1.5 doped CeO₂ are very promising interconnecting ceramics for solid oxide fuel cells (SOFCs).     

Accessing the second electron capacity of MnO2 by exploring complexation and intercalation reactions in energy dense alkaline batteries

MnO₂ is one of the safest and most abundant electrochemical materials available. It exhibits polymorphism, which has been exploited in many applications especially batteries. In alkaline batteries, γ-MnO₂ is widely used because its proton insertion reaction yields one e^? (1st) per Mn at high voltages. It also gives a second (2nd) e^? during dissolution-precipitation reactions that occur at lower voltages than the proton insertion; however, these 2nd e^? reactions are highly irreversible. In this communication, we explore the reversibility of the 2nd e^? reactions with bismuth oxide (Bi₂O₃) and copper (Cu) additives, and cycling in specific potential regions where δ-MnO₂ is the polymorph synthesized electrochemically in-situ from γ-MnO₂. The use of Bi₂O₃ and Cu add complexation and intercalation reactions, where presence and/or electrochemical activation of both are essential for reversibility and for capacity retention (50–100%). Attaining 300–610Wh kg^?1 against a zinc anode is possible for these batteries, which could promote its use for many applications.     

Investigation of the oxygen reduction reaction on the carbon electrodes loaded with MnO2 catalyst

The oxygen reduction reaction has been studied on gas diffusion electrodes made with various activated carbon materials and on the edge/basal orientations of pyrolitic graphite. A MnO₂ catalyst was loaded on all carbon surfaces. The MnO₂ catalyst demonstrated significant catalytic activity for the oxygen reduction reaction. The specific catalytic activity was found to relate to the concentration of the edge orientation of carbon materials loaded with MnO₂ catalyst. The higher the percentage of edge orientations, the higher the specific catalytic activity would be. MnO₂ may not participate in the reduction of O₂, but catalyze the disproportionation of HO₂⁻.     

Direct synthesis of MgH2 nanofibers from waste Mg

In this paper, we present a new route for recycling waste Mg with the production of MgH₂ nanofiber, which is a promising candidate for energy storage and conversion used in Li-ion batteries and H₂ storing technologies. Highly pure MgH₂ nanofibers were successfully manufactured from Mg-alloy swarfs (AM60B and AZ91D) via the hydriding chemical vapor deposition (HCVD) process. The possibility of recycling all types of Mg scraps using the HCVD process was demonstrated, which requires only a pretreatment of obtaining a dried raw material. The HCVD method shows many advantages over other methods in the synthesis of MgH₂: it can produce MgH₂ at low cost using waste Mg as the raw material, requires less energy, offers a high-purity product, and enables control over the growth of nanosized structures.     

Electrochemically-tuned luminescence of a [Ru(bpy)2(tatp)]-sensitized TiO2 anode and its applications to photo-stimulated guanine/H2O2 fuel cells

A phenazine-containing Ru(II) complex [Ru(bpy)₂(tatp)]²⁺ (bpy = 2,2′-bipyridine and tatp = 1,4,8,9-tetra-aza-triphenylene) is first applied to a modification of the nano-TiO₂/indium-tin oxide (ITO) electrode by the method of repetitive voltammetric sweeping. The resulting [Ru(bpy)₂(tatp)]²⁺-modified TiO₂ electrode shows two pairs of well-defined redox waves and excellent electrocatalytic activity for the oxidation of guanine. [Ru(bpy)₂(tatp)]²⁺ on TiO₂ surfaces exhibits intense absorbance and photoluminescence in visible region, revealed by absorption spectra, emission spectra and fluorescence microscope. While [Ru(bpy)₂(tatp)]²⁺-sensitized TiO₂ is functionalized as an anode to combine with a continuous wave green laser via an optical microscope, the luminescence of Ru(II)-based excited states can be enhanced by the oxidation of guanine. Furthermore, the [Ru(bpy)₂(tatp)]²⁺-sensitized TiO₂ electrode is used as photoanode and hemoglobin-modified single-walled carbon nanotubes (SWCNTs) as cathode for the elaboration of a photo-stimulated guanine/H₂O₂ fuel cell with a saturated KCl salt-bridge. It becomes evident that the photo-stimulated fuel cell performance depends strongly on the excited states of Ru(II) complex-sensitized anodes as well as the electrocatalytic oxidation of guanine. This study provides an electrochemically-tuned luminescence method for better evaluating contributions of the sensitizer excited states to photo-stimulated fuel cells.     

Development of Ag-(BaO)0.11(Bi2O3)0.89 composite cathodes for intermediate temperature solid oxide fuel cells

The electrochemical performances of Ag-(BaO)_0.11(Bi₂O₃)_0.89 (BSB) composite cathodes on Ce_0.8Sm_0.2O_1.9 electrolytes have been investigated for intermediate temperature solid oxide fuel cells (ITSOFCs) using ac impedance spectroscopy from 500 to 700 °C. Results indicate that the electrochemical properties of these composites are quite sensitive to the composition and the microstructure of the cathode. The optimum BSB addition (50% by volume) to Ag resulted in about 20 times lower area specific resistance (ASR) at 650 °C. The ASR values for the Ag50-BSB and Ag cathodes were 0.32 and 6.5 Ω cm² at 650 °C, respectively. The high performances of Ag-BSB cathodes are determined by the high catalytic activity for oxygen dissociation and ionic conductivity of BSB, and by the excellent catalytic activity for oxygen reduction of silver. The maximum power density of the Ag50-BSB cathode was 224 mWcm⁻² at 650 °C, which classify this composite as a promising material for ITSOFC.     

Convenient storage of concentrated hydrogen peroxide as a CaO2·2H2O2(s)/H2O2(aq) slurry for energy storage applications

Prior investigations have proposed, and successfully implemented, a stand-alone supply of aqueous hydrogen peroxide for use in fuel cells. An apparent obstacle for considering the use of aqueous hydrogen peroxide as an energy storage compound is the corrosive nature of the nominally required 50 wt.% maximum concentration. Here we propose storage of concentrated hydrogen peroxide in a high weight percent solid slurry, namely the equilibrium system of CaO₂·2H₂O₂(s)/H₂O₂(aq), that mitigates much of the risk associated with the storage of such high concentrations. We have prepared and studied surrogate slurries of calcium hydroxide/water that are assumed to resemble the peroxo compound slurries. These slurries have the consistency of a paste rather than a distinct two-phase (liquid plus solid) system. This paste-like property of the prepared surrogates enable them to be contained within a 200 lines-per-inch. (LPI) nickel mesh screen (33.6% open area) with no solids leakage, and only liquid transport driven by an adsorbent material is placed in physical contact on the exterior of the screen. This hydrogen peroxide slurry approach suggests a convenient and safe mechanism of storing hydrogen peroxide for use in, say, vehicle applications. This is because fuel cell design requires only aqueous hydrogen peroxide use, that can be achieved using the separation approach utilizing the screen material here. This proposed method of storage should mitigate hazards associated with unintentional spills and leakage issues arising from aqueous solution use.     

Synthesis and electrochemical studies of the 4 V cathode, Li(Ni2/3Mn1/3)O2

Layered Li(Ni_2/3Mn_1/3)O₂ compounds are prepared by freeze-drying, mixed carbonate and molten salt methods at high temperature. The phases are characterized by X-ray diffraction, Rietveld refinement, and other methods. Electrochemical properties are studied versus Li-metal by charge–discharge cycling and cyclic voltammetry (CV). The compound prepared by the carbonate route shows a stable capacity of 145 (±3) mAh g⁻¹ up to 100 cycles in the range 2.5–4.3 V at 22 mA g⁻¹. In the range 2.5–4.4 V at 22 mA g⁻¹, the compound prepared by molten salt method has a stable capacity of 135 (±3) mAh g⁻¹ up to 50 cycles and retains 96% of this value after 100 cycles. Capacity-fading is observed in all the compounds when cycled in the range 2.5–4.5 V. All the compounds display a clear redox process at 3.65–4.0 V that corresponds to the Ni^2+/3+–Ni^3+/4+ couple.