Influence of operation conditions on direct NaBH4/H2O2 fuel cell performance

Although hydrogen fuel cells have attracted so much attentions in these years because of the application prospect in electric vehicles, some obstacles have not been solved yet, among which hydrogen storage is one of the biggest. Direct borohydride fuel cell (DBFC) is another choice without hydrogen storage problem because borohydride is used as reactant directly in the fuel cell. In this paper, DBFC performance under different operation conditions was studied including electrolyte membrane type, operation temperature, borohydride concentration, supporting electrolyte and oxidant. Results showed that, with Pt/C and MnO₂ as anode and cathode electrocatalyst, respectively, Nafion^® 117 membrane as electrolyte, 1.0 M, 3.0 M and 6.0 M NaBH₄ and H₂O₂ solution in NaOH as reactant solution, 80 °C operation, the peak power density could reach 130 mW/cm².     

Direct NaBH4/H2O2 fuel cells

A fuel cell (FC) using liquid fuel and oxidizer is under investigation. H₂O₂ is used in this FC directly at the cathode. Either of two types of reactant, namely a gas-phase hydrogen or an aqueous NaBH₄ solution, are utilized as fuel at the anode. Experiments demonstrate that the direct utilization of H₂O₂ and NaBH₄ at the electrodes results in >30% higher voltage output compared to the ordinary H₂/O₂ FC. Further, the use of this combination of all liquid fuels, provides numerous advantages (ease of storage, reduced pumping requirements, simplified heat removal, etc.) from an operational point of view. This design is inherently compact compared to other cells that use gas phase reactants. Further, regeneration is possible using an electrical input, e.g. from power lines or a solar panel. While the peroxide-based FC is ideally suited for applications such as space power where air is not available and a high energy density fuel is essential, other distributed and mobile power uses are of interest.     

NaBH4/H2O2 fuel cells for air independent power systems

The performance and characteristics of direct sodium-borohydride/hydrogen-peroxide (NaBH₄/H₂O₂) fuel cells are studied in the context of potential applications for air independent propulsion for outer space and underwater. Due to the existence of ocean (sea) water as a natural heat sink, this new fuel cell technology is best suited for underwater propulsion/power systems for small scale high performance marine vehicles. The characteristics of such a power system are compared to other options, specifically for the underwater scenario. The potential of this fuel cell is demonstrated in laboratory experiments. Power density over 1.5 W cm⁻², at 65 °C and ambient pressure, have been achieved with the help of some unique treatments of the fuel cell. One such treatment is an in-situ electroplating technique, which results in electrodes with power density 20–40% higher, than that of the electrodes produced by the ordinary ex-situ electroplating method. This unique process also makes repair or reconditioning of the fuel cell possible and convenient.     

A novel direct ethanol fuel cell with high power density

A new type of direct ethanol fuel cell (DEFC) that is composed of an alkaline anode and an acid cathode separated with a charger conducting membrane is developed. Theoretically it is shown that the voltage of this novel fuel cell is 2.52 V, while, experimentally it has been demonstrated that this fuel cell can yield an open-circuit voltage (OCV) of 1.60 V and a peak power density of 240 mW cm⁻² at 60 °C, which represent the highest performance of DEFCs that has so far been reported in the open literature.     

Catalytic behavior of Co3O4 in electroreduction of H2O2

Spinel structure Co₃O₄ nanoparticles with an average diameter of around 17 nm were prepared and evaluated as electrocatalysts for H₂O₂ reduction. Results revealed that Co₃O₄ exhibits considerable activity and good stability for electrocatalytic reduction of H₂O₂ in 3 M NaOH solution. The reduction occurs mainly via the direct pathway when H₂O₂ concentration is lower than 0.5 M. An Al-H₂O₂ semi fuel cell using Co₃O₄ as cathode catalyst was constructed and tested at room temperature. The fuel cell displayed an open circuit voltage of 1.45 V and a peak power density of 190 mW cm⁻² at a current density of 255 mA cm⁻² operating with a catholyte containing 1.5 M H₂O₂. This study demonstrated that Co₃O₄ nanoparticles are promising cathode catalysts, in place of precious metals, for fuel cells using H₂O₂ as oxidant.     

Fully-integrated micro PEM fuel cell system with NaBH4 hydrogen generator

A fully-integrated micro PEM fuel cell system with a NaBH₄ hydrogen generator was developed. The micro fuel cell system contained a micro PEM fuel cell and a NaBH₄ hydrogen generator. The hydrogen generator comprised a NaBH₄ reacting chamber and a hydrogen separating chamber. Photosensitive glass wafers were used to fabricate a lightweight and corrosion-resistant micro fuel cell and hydrogen generator. All of the BOP such as a NaBH₄ cartridge, a micropump, and an auxiliary battery were fully integrated. In order to generate stable power output, a hybrid power management operating with a micro fuel cell and battery was designed. The integrated performance of the micro PEM fuel cell with NaBH₄ hydrogen generator was evaluated under various operating conditions. The hybrid power output was stably provided by the micro PEM fuel cell and auxiliary battery. The maximum power output and specific energy density of the micro PEM fuel cell system were 250 mW and 111.2 W h/kg, respectively.     

A flexible on-fiber H2O2 microfluidic fuel cell with high power density

To promote the simplification and integration of membraneless microfluidic fuel cell (MMFC) system and combine with flexible portable devices, a flexible on-fiber MMFC exploiting H₂O₂ as sole reactant is presented, eliminating the separation requirement of fuel and oxidant. Nickel (Ni) nano-particles and Prussian blue with multiwalled carbon nanotube (PB-MWCNT) are coated on hydrophilic braided carbon fibers (BCFs) to serve as the anode and cathode, respectively. The three-dimensional (3D) flow-through anode and cathode with a wealth of exposed electroactive sites improve reactant mass transfer. The anode and cathode are respectively wound on both sides of the middle cotton thread-based flow channel for separation. Under the combination of capillary force and gravity, reactants flow continuously through the fiber-based microchannels without external pumps. Importantly, the H₂O₂ MMFC achieves the highest maximum power density (MPD) of 14.41 mW cm^?2 so far in one-chamber or single-stream H₂O₂ fuel cells. Besides, no serious deterioration in the power-generation performance is observed in complex practical operating conditions including bending with various angles, repeated folding and dropping. Three presented flexible MMFCs are connected to power a handheld calculator, indicating the tremendous potential of developing micro power supplies based on abundant flexible materials as well as green and sustainable energy.     

Engineering of the bipolar stack of a direct NaBH4 fuel cell

When a fuel cell (FC) utilizes liquid fuels directly, a few complications arise due to the conductance or the potential conductance of the fuel. Fuel cell stacks are typically designed in a bipolar fashion so that the voltage of individual cells can be added up in series to give an adequate and convenient output voltage. The conductivity of fuels brings about two risks if the bipolar stack is not properly designed and engineered. On one hand, the conductive liquid fuel may short circuit the neighboring cells of a bipolar FC stack with traditional integrated fuel manifolds. On the other hand, the conductive fuel may pass a high voltage to some parts of the cell through an ordinary manifold, causing excessive corrosion. These issues need to be addressed through a cell-isolation fuel distribution network (FDN). The function of such an FDN is to increase the shunting resistance of neighboring cells, so as to maintain a reasonable open circuit voltage. Also, the presence of a gas phase in the liquid fuel during cell operation affects fuel circulation and therefore needs to be considered in the FDN design. On the plus side, a liquid fuel, in contact with high surface area FC electrodes, functions as a super-capacitor, giving the FC an excellent pulse overload capacity. Also the fuel itself is a fair coolant, enabling high power density at minimal increase in stack weight. These considerations are applied to a kilowatt NaBH₄/H₂O₂ fuel cell stack to generate the desired operational characteristics.     

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.     

Electrochromic properties of heat-treated thin films of CeO2–TiO2–ZrO2 prepared by sol–gel route

CeO₂–TiO₂–ZrO₂ thin films were prepared using the sol–gel process and deposited on glass and ITO-coated glass substrates via dip-coating technique. The samples were heat treated between 100 and 500 °C. The heat treatment effects on the electrochromic performances of the films were determined by means of cyclic voltammetry measurements. The structural behavior of the film was characterized by atomic force microscopy and X-ray diffraction. Refractive index, extinction coefficient, and thickness of the films were determined in the 350–1000 nm wavelength, using nkd spectrophotometry analysis.Heat treatment temperature affects the electrochromic, optical, and structural properties of the film. The charge density of the samples increased from 8.8 to 14.8 mC/cm², with increasing heat-treatment temperatures from 100 to 500 °C. It was determined that the highest ratio between anodic and cathodic charge takes place with increase of temperature up to 500 °C.     

Nanostructured silver catalyzed nickel foam cathode for an aluminum–hydrogen peroxide fuel cell

A nanostructured Ag catalyzed nickel foam cathode for an aluminum–hydrogen peroxide fuel cell was prepared using an electrodeposition technique. SEM images show that Ag nano-islands, about 2–3 μm in length and 100–200 nm in width are aligned on the surface of the Ni foam substrate. The composition of the catalyst layer of the cathode was examined by XRD. Electrochemical performance and stability of the cathode for the reduction of hydrogen peroxide in aluminum–hydrogen peroxide fuel cell were studied.     

Thermal cycle stability of BaO–B2O3–SiO2 sealing glass

Thermal cycle stability is very important for glass seals in planar solid oxide fuel cell (pSOFC) applications. In the present study, thermal cycle stability of a thermally stable sealing glass is investigated using a sealing fixture from 150 °C to 700 °C. SS410 alloy with the TEC (thermal expansion coefficient) of 12.2 × 10⁻⁶ K⁻¹ (room temperature to 700 °C) is used to evaluate the effect of TEC mismatch on the thermal cycle stability. The leak rates increase with thermal cycles and appear to be two different stages. Microstructure examinations are performed to investigate the degradation mechanism of the thermal cycle stability. It is found that the sealing glass interacts chemically with the SS410 alloy and the formation of BaCrO₄ new phase results in the rapid increase of the leak rates.     

Sm0.2(Ce1−xTix)0.8O1.9 modified Ni-yttria-stabilized zirconia anode for direct methane fuel cell

Sm_0.2(Ce_1−xTi_x)_0.8O_1.9 (SCTx, x = 0-0.29) modified Ni-yttria-stabilized zirconia (YSZ) has been fabricated and evaluated as anode in solid oxide fuel cells for direct utilization of methane fuel. It has been found that both the amount of Ti-doping and the SCTx loading level in the anode have substantial effect on the electrochemical activity for methane oxidation. Optimal anode performance for methane oxidation has been obtained for Sm_0.2(Ce_0.83Ti_0.17)_0.8O_1.9 (SCT0.17) modified Ni-YSZ anode with SCT0.17 loading of about 241 mg cm⁻² resulted from four repeated impregnation cycles. When operating on humidified methane as fuel and ambient air as oxidant at 700 °C, single cells with the configuration of SCT0.17 modified Ni-YSZ anode, YSZ electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O₃-Sm_0.2Ce_0.8O_1.9 (LSCF-SDC) composite cathode show the polarization cell resistance of 0.63 Ω cm² under open circuit conditions and produce a peak power density of 383 mW cm⁻². It has been revealed that the coated Ti-doped SDC on Ni-YSZ anode not only effectively prevents the methane fuel from directly impacting on the Ni particles, but also enhances the kinetics of methane oxidation due to an improved oxygen storage capacity (OSC) and redox equilibrium of the anode surface, resulting in significant enhancement of the SCTx modified Ni-YSZ anode for direct methane oxidation.     

Sr2CoMoO6 anode for solid oxide fuel cell running on H2 and CH4 fuels

The double perovskite Sr₂CoMoO_6−δ was investigated as a candidate anode for a solid oxide fuel cell (SOFC). Thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) showed that the cation array is retained to 800 °C in H₂ atmosphere with the introduction of a limited concentration of oxide-ion vacancies. Stoichiometric Sr₂CoMoO₆ has an antiferromagnetic Néel temperature T_N ≈ 37 K, but after reduction in H₂ at 800 °C for 10 h, long-range magnetic order appears to set in above 300 K. In H₂, the electronic conductivity increases sharply with temperature in the interval 400 °C < T < 500 °C due to the onset of a loss of oxygen to make Sr₂CoMoO_6−δ a good mixed oxide-ion/electronic conductor (MIEC). With a 300-μm-thick La_0.8Sr_0.12Ga_0.83Mg_0.17O_2.815 (LSGM) as oxide-ion electrolyte and SrCo_0.8Fe_0.2O_3−δ as the cathode, the Sr₂CoMoO_6−δ anode gave a maximum power density of 1017 mW cm⁻² in H₂ and 634 mW cm⁻² in wet CH₄. A degradation of power in CH₄ was observed, which could be attributed to coke build up observed by energy dispersive spectroscopy (EDS).     

La2O3-Al2O3-B2O3-SiO2 glasses for solid oxide fuel cell applications

La₂O₃-Al₂O₃-B₂O₃-SiO₂ glasses free of alkaline earth metals were prepared in this study for SOFC applications to relieve the poison caused by BaCrO₄ or SrCrO₄ formation. The apparent densities, coefficients of thermal expansion (CTE), and softening points of the La₂O₃-Al₂O₃-SiO₂-B₂O₃ glasses prepared in this study ranged respectively from 3.24 to 4.54 g/cm³, 4.1 to 8.1 ppm/°C, and 912 to 937 °C, depending on the glass composition. The CTE value dropped with the rise in SiO₂ content and escalated with increase in La₂O₃ content. Crystallization of La_9.51(SiO_1.0404)₆O₂ and La_4.67(SiO₄)₃O was observed in part of the glasses after soaking at 800 °C. Two CTE modifiers, MgO and SDC, effectively increased the CTE of La₂O₃-Al₂O₃-SiO₂-B₂O₃ glasses. The composites of selected La₂O₃-Al₂O₃-B₂O₃-SiO₂ glasses and SDC additive on the YSZ substrate were evaluated for use as sealing materials of solid oxide fuel cells (SOFCs). Results indicated that the leakage rates for the composites of A07 glass and 60-70 vol% SDC on the YSZ plate read less than 0.02 (sccm/cm)(kg/cm²) per min at 800 °C. This property seems highly promising for ensuring long-term stability of the sealing materials for SOFC applications.     

Sol–gel derived nanocrystalline CeO2–TiO2 coatings for electrochromic windows

Mixed CeO₂–TiO₂ coatings synthesized by sol–gel spin coating process using mixed organic–inorganic Ti(OC₃H₇)₄ and CeCl₃·7H₂O precursors with different Ce/Ti mole ratios were investigated by a wide range of characterization techniques. The attempts were directed towards achieving coatings with high transparency in the visible region and good electrochemical properties. Elucidation of the structural and optical features of the films yielded information on the aspects relevant to their usage in transmissive electrochromic devices. The films have been found to exhibit properties for counter electrode in electrochromic smart windows in which they are able to retain their transparency under charge insertion, high enough for practical uses. The high optical modulation and fastest switching for WO₃ film in the device configuration with the Ce/Ti (1:1) film is interpreted in terms of conducive microstructural changes induced by addition of TiO₂ in an amount equivalent to CeO₂.     

High performance of proton-conducting solid oxide fuel cell with a layered PrBaCo2O5+δ cathode

A dense BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte is fabricated on a porous anode by in situ drop-coating method which can lead to extremely thin electrolyte membrane (10 μm in thickness). The layered perovskite structure oxide PrBaCo₂O_5+δ (PBCO) is synthesized by auto ignition process and initially examined as a cathode for proton-conducting IT-SOFCs. The electrical conductivity of PrBaCo₂O_5+δ (PBCO) reaches the general required value for the electrical conductivity of cathode absolutely. The single cell, consisting of PrBaCo₂O_5+δ (PBCO)/BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY)/NiO-BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) structure, is assembled and tested from 600 to 700 °C with humidified hydrogen (3% H₂O) as the fuel and air as the oxidant. An open-circuit potential of 1.01 V and a maximum power density of 545 mW cm⁻² at 700 °C are obtained for the single cell, and a low polarization resistance of the electrodes of 0.15 Ω cm² is achieved at 700 °C.     

Ti4O7 supported IrOx for anode reversal tolerance in proton exchange membrane fuel cell

Fuel starvation can occur and cause damage to the cell when proton exchange membrane fuel cells operate under complex working conditions. In this case, carbon corrosion occurs. Oxygen evolution reaction (OER) catalysts can alleviate carbon corrosion by introducing water electrolysis at a lower potential at the anode in fuel shortage. The mixture of hydrogen oxidation reaction (HOR) and unsupported OER catalyst not only reduces the electrolysis efficiency, but also influences the initial performance of the fuel cell. Herein, Ti₄O₇ supported IrO_x is synthesized by utilizing the surfactant-assistant method and serves as reversal tolerant components in the anode. When the cell reverse time is less than 100 min, the cell voltage of the MEA added with IrO_x/Ti₄O₇ has almost no attenuation. Besides, the MEA has a longer reversal time (530 min) than IrO_x (75 min), showing an excellent reversal tolerance. The results of electron microscopy spectroscopy show that IrO_x particles have a good dispersity on the surface of Ti₄O₇ and IrO_x/Ti₄O₇ particles are uniformly dispersed on the anode catalytic layer. After the stability test, the Ti₄O₇ support has little decay, demonstrating a high electrochemical stability. IrO_x/Ti₄O₇ with a high dispersity has a great potential to the application on the reversal tolerance anode of the fuel cell.     

LaNi0.8Co0.2O3 as a cathode catalyst for a direct borohydride fuel cell

A perovskite-type oxide LaNi_0.8Co_0.2O₃ is prepared as a direct borohydride fuel cell (DBFC) cathode catalyst. Its electrochemical properties are studied by cyclic voltammetry. The results demonstrate that LaNi_0.8Co_0.2O₃ exhibits excellent electrochemical activity with respect to the oxygen reduction reaction (ORR) and good tolerance of BH₄⁻ ions. Maximum power densities of 114.5 mW cm⁻² at 30 °C and 151.3 mW cm⁻² at 62 °C are obtained, and good stability (300-h stable performance at 20 mA cm⁻²) is also exhibited, which shows that such perovskite-type oxides as LaNi_0.8Co_0.2O₃ can be excellent catalysts for DBFCs.