Interleaved soft-switched active-clamped L–L type current-fed half-bridge DC–DC converter for fuel cell applications

In this paper, an interleaved soft-switched active-clamped L–L type current-fed half-bridge isolated dc–dc converter has been proposed. The L–L type active-clamped current-fed converter is able to maintain zero-voltage switching (ZVS) of all switches for the complete operating range of wide fuel cell stack voltage variation at full load down to light load conditions. Active-clamped circuit absorbs the turn-off voltage spike across the switches. Half-bridge topology maintains higher efficiency due to lower conduction losses. Soft-switching permits higher switching frequency operation, reducing the size, weight and cost of the magnetic components. Interleaving of the two isolated converters is done using parallel input series output approach and phase-shifted modulation is adopted. It reduces the input current ripple at the fuel cell input, which is required in a fuel cell system and also reduces the output voltage ripples. In addition, the size of the magnetic/passive components, current rating of the switches and voltage ratings of the rectifier diodes are reduced.     

Nonlinear stability analysis and a new design methodology for a PEM fuel cell fed DC–DC boost converter

The dynamical behaviour of a fuel cell feeding a boost converter is studied in this paper. A nonlinear model of the combined system is derived including the effect of the switching action of the converter. Using Filippov's theory, it is possible to analytically study the bifurcation patterns of the system and to demonstrate that the system loses stability through a period doubling bifurcation. To overcome this instability, we inject a high frequency sinusoidal signal into the system that forces the system to remain stable while at the same time retaining its basic slow scale properties (like the steady state error). This controller is simple to implement and does not require any special hardware. The stability analysis and new controller design method presented in this paper allow for the re-design of the converter to stabilize circuit operation with a substantially reduced inductor size, reducing the size and cost of the converter while maintaining its average currents and voltages and other circuit steady-state behaviour characteristics. The results are confirmed by using numerical and analytical tools.     

High voltage step-up integrated double Boost–Sepic DC–DC converter for fuel-cell and photovoltaic applications

In this paper, an integrated double boost SEPIC (IDBS) converter is proposed as a high step-up converter. The proposed converter utilizes a single controlled power switch and two inductors and is able to provide high voltage gain without extreme switch duty-cycle. The two inductors can be coupled into one core for reducing the input current ripple without affecting the basic DC characteristic of the converter. Moreover, the voltage stresses across all the semiconductors are less than half of the output voltage. The reduced voltage stress across the power switch enables the use of a lower voltage and R_DS-ON MOSFET switch, which will further reduce the conduction losses. Whereas, the low voltage stress across the diodes allows the use of Schottky rectifiers for alleviating the reverse-recovery current problem, leading to a further reduction in the switching and conduction losses. A detailed circuit analysis is performed to derive the design equations. A design example for a 100-W/240 V_dc with 24 V_dc input voltage is provided. The feasibility of the converter is confirmed with results obtained from simulation and an experimental prototype.     

A high-flexibility DC load for fuel cell and solar arrays power sources based on DC–DC converters

In this paper, a flexible DC load to test and evaluate current–voltage characteristics of fuel cells stacks and photovoltaic modules based on DC–DC converters is proposed. The load features are simple structure, scalability, low cost, and its possibility to emulate an arbitrary load profile. The measure of the desired characteristics of fuel cells and photovoltaic modules further includes high speed of response and high fidelity. A comparison between conventional methods and the proposed one is also provided. Experimental results show the usefulness of the DC load proposed.     

FC/UC hybridization for dynamic loads with a novel double input DC–DC converter topology

Due to the fact that the environmental issues have become more serious recently, interest in renewable energy systems, such as, fuel-cells (FCs) has increased steadfastly. Among many types of FCs, proton exchange membrane FC (PEMFC) is one of the most promising power sources due to its advantages, such as, low operation temperature, high power density and low emission. However, using sole PEMFC for dynamic loads may not be feasible to satisfy the peak demand changes. Therefore, hybridizing PEMFC and an energy storage system (ESS) decreases the FC cost and improves its performance and life. Ultra-capacitor (UC) is the most powerful candidate to hybridize with PEMFC for dynamic loads. The DC–DC converter is the key enabling technology for hybridization of PEMFC and UC. Generally, the efficiency and performance of hybridization is largely limited by the converter topology employed for the mentioned hybridization. Integrating each source (PEMFC and UC) with a DC–DC converter is not feasible in terms of cost, performance, and control. Due to the above mentioned reasons, an attractive converter topology which can combine PEMFC and UC is strongly required. In this regard, the objective of this study is to design and simulate a novel double input DC–DC converter based on current additivity concept, in order to combine two different types of energy systems (PEMFCs and UCs).     

Design and analysis of a high frequency DC–DC converters for fuel cell and super-capacitor used in electrical vehicle

This work focuses on the application of the high frequency DC–DC converters used in electric vehicles. Two converters are necessary. The first converter is interposed between the fuel cell and the DC–AC inverter. It is unidirectional. The second one is used as interface between the ultra-capacitor and the DC–AC inverter. It allows the bidirectional of the power transfer. Each converter is composed of two full bridges, LC resonant filter and two planar transformers. The use of high frequency transformer allows to minimize the size and weight of the converter, produce a higher voltage in secondary side from input voltage (fuel cell or super-capacitor) and isolate the full bridges. The control strategy of the converters is the phase shift. The converters have been designed, realised and controlled by an FPGA board. To demonstrate the converters feasibility, two converters are implemented and tested. The switching frequency of two converters is 20 kHz. The first converter has a 24-V input and 200 V/1.2 kW output. But, the second converter has a 12.5-V input and 100 V/400 W output.     

MPPT controller for an interleaved boost dc–dc converter used in fuel cell electric vehicles

Fuel cell electric vehicle (FCEV) has recently attracted increasing research interest. This paper investigates the performances of MPPT-FC generators supplying electric vehicle power train through an interleaved boost DC/DC converter (IBC). The accent is made on forcing the FC generator to operate at its maximum power point by using perturb and observe (P&O) algorithm integrated to the IBC control. However, the MPPT-FC control creates rapid changes in the power output from the fuel cell, which cause serious life shortening, severe cell degradation, and decrease the system efficiency. To overcome these shortcomings, the control of air generation system was designed to improve the power quality and to prevent fuel starvation phenomenon during rapid power transitions. The work involves the modeling and the simulation of the fuel cell power train in the vehicular application using the experimental data obtained in previous works. The experimental part of the proposed FCEV is based on a low-cost, low-power consumption microcontroller, which controls the IBC and performs the MPPT-FC operation. A microcontroller is used to measure the FC output power and to change the duty ratio of the IBC control signals.     

A novel direct methanol fuel cell with sol–gel flux phase

The sol–gel flux phase of direct methanol fuel cell is prepared by the modified sol–gel method with starting materials of Na₂SiO₃ or Si(OCH₃)₄, methanol and sulfuric acid, and characterized by SEM. The methanol permeability and electrochemical characteristics of the sol–gel flux phase are investigated. The mass transportation mechanism and the process of methanol has been changed by the porous structure of the sol–gel flux phase. The methanol permeability of the sol–gel flux phase decreases more than 90% compared with the liquid flux phase of 1 mol L^?1 CH₃OH and 1 mol L^?1 H₂SO₄. A novel direct methanol fuel cell with sol–gel flux phase is designed. The power density of which is higher than that of the cell with liquid flux phase.     

A spatially resolved fuel cell stack model with gas–liquid slip phenomena for cold start simulations

This paper proposes a novel fuel cell stack model for system simulation of the cold start operation. The stack model features the option of spatial resolution in two directions and can simulate two-phase effects that are prone to occur during cold start. These effects include: the sorption process in the triple-phase zone between the membrane and the catalyst layer; the condensation, freezing and evaporation in the porous layers; the two-phase flow in the gas channels with a gas–liquid slip model. Compared to a 0D lumped model, the proposed stack model provides a more detailed view of the onset of freezing and liquid saturation. In a freeze start simulation from ?20 °C, the Rapid-Warmup-Operation published by Naganuma et al. [1] is applied and demonstrated as an effective way to quickly elevate stack temperature to +5 °C within 30 s. The stack model illustrates in detail the physical phenomena in the cold start and can be further integrated into a powertrain model for drive cycle analysis.     

New method for low temperature fabrication of Ni–Al alloy powder for molten carbonate fuel cell applications

Lump-free Ni-5 wt% Al alloy powder was successfully prepared using an AlCl₃ activator at 400 °C under vacuum. The AlCl₃ activator served as the catalyst, lowering the fabrication temperature by 1000 °C compared with the temperature required for the conventional process. The Ni–Al alloy was formed by the following steps: the formation of NiAl by the reaction of the Ni surface with AlCl₂ or AlCl produced by the reaction between Al and AlCl₃, the formation of Ni₃Al by Al diffusion and reaction, and the formation of a Ni–Al solid solution by Al diffusion into the Ni matrix until the solubility limitation was reached. Although lowering the alloying temperature lengthens the reaction time, the time could be reduced by controlling the amount of AlCl₃. A single cell test and a creep test were also conducted using a green sheet of as-prepared Ni–Al alloy powder as an anode of a molten carbonate fuel cell (MCFC).     

Li2TiO3–LaSrCoFeO3 semiconductor heterostructure for low temperature ceramic fuel cell electrolyte

Semiconductor-based electrolytes have significant advantages than conventional ionic electrolyte fuel cells, especially for high ionic conductivity and power outputs at low temperatures (<600 °C). This work reports a p-n heterojunction composite electrolyte developed by a p-type La_0.8Sr_0.2Co_0.8Fe_0.2O_3-δ (LSCF) and n-type Li₂TiO₃ (LTO). It achieved a power output of 350 mW cm^?2 at 550 °C using LSCF-LTO heterostructure as the electrolyte. On the other hand, pure LSCF and Li₂TiO₃ were made as the fuel cell electrolyte as well. The former resulted immediately a short circuiting problem and exhibited no device voltage because of high electron (hole) conductivity. While the Li₂TiO₃ can reach an open circuit voltage (OCV) but deliver too low power output, 37 mW cm^?2 at 550 °C. Scanning Electron Microscope (SEM) combined with High-Resolution Transmission Electron Microscope (HR-TEM) clearly proved the formation of heterogeneous interface. Also, Fourier Transform Infrared Spectroscopy (FTIR) was performed to demonstrate the functional group of the synthesized materials. The results demonstrate clearly the semiconductor heterostructure effect. By adjusting apriority composition of the n-type and p-type components, electronic conduction is well suppressed in the membrane electrolyte. Meanwhile, by constructing p-n heterostructure and build-in field, we have succeeded in high ionic conductivity, high current and power outputs for the low temperature fuel cells. The results are interesting in general that to construct a p-n heterostructure electrolyte can be an effective and common way in developing low temperature ceramic fuel cells.     

Characterization of high strength and high toughness Ni–Mo–Cr low alloy steels for nuclear application

The reactor pressure vessels of PWRs have mostly been made of SA508 Grade 3 (Class 1) low alloy steels which have revealed moderate mechanical properties and a moderate radiation resistance for a 40 or 60 year operation. The specified minimum yield strength of the material is 345 MPa with a ductile–brittle transition temperature of about 0 °C. While other materials, most of which are non-ferrous alloys or high alloyed steels for a higher temperature application, are being developed for the Generation-4 reactors, low alloy steels with a higher strength and toughness can help to increase the safety and economy of the advanced PWR systems which will be launched in the near future. The ASME specification for SA508 Grade 4N provides a way to increase both the strength and toughness by a chemistry modification, especially by increasing the Ni and Cr contents. However, a higher strength steel has a deficiency due to a lack of operating data for nuclear power plants. In this study, experimental heats of SA508 Grade 4N steels with different chemical compositions were characterized mechanically. The preliminary results for an irradiation embrittlement and the HAZ properties are discussed in addition to their superior baseline properties.     

FeOOH–CoS composite catalyst with high catalytic performances for oxygen evolution reaction at high current density

Water electrolysis is an efficient approach for high-purity hydrogen production. However, the anodic sluggish oxygen evolution reaction (OER) always needs high overpotential and thus brings about superfluous electricity cost of water electrolysis. Therefore, exploiting highly efficient OER electrocatalysts with small overpotential especially at high current density will undoubtedly boost the development of industrial water electrolysis. Herein, we used a simple hydrothermal method to prepare a novel FeOOH–CoS nanocomposite on nickel foam (NF). The as-prepared FeOOH–CoS/NF catalyst displays an excellent OER performance with extremely low overpotentials of 306 and 329 mV at 500 and 1000 mA cm⁻² in 1.0 M KOH, respectively. In addition, the FeOOH–CoS/NF catalyst can maintain excellent catalytic stability for more than 50 h, and the OER catalytic activity shows almost no attenuation no matter after 1000 repeated CV cycles or 50 h of stability test. The high catalytic activity and stability have exceeded most non-noble metal electrocatalysts reported in literature, which makes the FeOOH–CoS/NF composite catalyst have promising applications in the industrial water electrolysis.     

Formic acid–Formate blended solution: A new fuel system with high oxidation activity

Formic acid and formate have been proven to be among the most promising fuels for direct liquid fuel cells. To take advantage of both formic acid and formate, the oxidation activity of a formic acid–formate blended solution was studied using cyclic voltammetry, Tafel polarization measurements, and chromoamperometry. With increased concentration of formate, the oxidation potential of formic acid shifted to the negative direction, and the peak current significantly increased, suggesting a greatly enhanced oxidation activity. The transition of formic acid from an indirect to a direct oxidation route was also observed. The enhanced oxidation activity indicated that a formic acid–formate blended solution is a promising fuel system for direct liquid fuel cells. Hence, a new concept of fuel cell, i.e., direct formic acid–formate blended fuel cell, was proposed.     

A study on novel combined processes for preparation of high performance Pt–Co/C electrocatalyst for oxygen reduction in PEM fuel cell

The preparation of a Pt–Co/C electrocatalyst for the oxygen reduction reaction in PEM fuel cells was achieved via a novel combined process of impregnation and seeding. The effect of seeding and non-seeding approaches on the morphologies and activities of the electrocatalyst was explored. The results indicated that the seeding or non-seeding approaches provided the similar results of Pt structure and phase composition in the Pt–Co/C electrocatalyst. However, the seeding approach provided a more uniform dispersion and smaller particle size of electrocatalyst compared with that of the non-seeding approach. Also, higher values of kinetic parameters including i₀, E₀, i_0.9V and E_10mA/cm2 were obtained in case of seeding electrocatalyst. Finally, the rotating disk electrode experimental results showed that the mechanism of oxygen reduction involved the four-electron pathway.     

Comparison of Cu–Mn and Mn–Co spinel coatings for solid oxide fuel cell interconnects

Electrophoretic deposition (EPD) of protective coatings on solid oxide fuel cell (SOFC) interconnects is an efficient method to mitigate ‘chromium poisoning’, which is a primary reason for degradation of fuel cell performance. Cu–Mn spinels and Mn–Co spinels are the most widely used materials for such coatings. In this study, four spinel coatings were examined; CuMn₂O₄, CuNi_0.2Mn_1.8O₄, MnCo₂O₄, and MnFe_0.34Co_1.66O₄. The coatings were evaluated on multiple criteria; including phase stability, microstructural stability, conductivity, Cr gettering ability, ability to act as a diffusion barrier to outward chromium and inward oxygen diffusion, and the ability to limit the increase in the area specific resistance (ASR) during high temperature oxidation exposures. The results showed that, while different coatings have best individual characteristics, overall CuNi_0.2Mn_1.8O₄ was the best candidate for the coatings operating in the intermediate temperature range due to its best sinterability, highest conductivity, lowest ASR, phase stability over the operational temperature range, lower cost and good resistance to outward chromium and inward oxygen diffusion.     

Development of trimetallic Ni–Cu–Zn anode for low temperature solid oxide fuel cells with composite electrolyte

Trimetallic alloys of Ni_0.6Cu_0.4−xZn_x (x = 0, 0.1, 0.2, 0.3, 0.4) have been investigated as promising anode materials for low temperature solid oxide fuel cells (SOFCs) with composite electrolyte. The alloys have been obtained by reduction of Ni_0.6Cu_0.4−xZn_xO oxides, which are synthesized by using the glycine–nitrate process. Increasing the Zn content x decreases the particle sizes of the oxides at a given sintering temperature. Fuel cells have been constructed using lithiated NiO as cathode and as-prepared alloys as anodes based on the composite electrolyte. Peak power densities are observed to increase with the increasing Zn addition concentration into the anode. The maximum power density of 624 mW cm⁻² at 600 °C, 375 mW cm⁻² at 500 °C has been achieved for the fuel cell equipped with Ni_0.6Zn_0.4 anode. A.c. impedance results show that the resistances dramatically decrease with increasing temperatures under open circuit voltage state. Both cathodic and anodic interfacial polarization resistances increase with the amplitude of applied DC voltage. Possible reaction process for H₂ oxidation reaction at anode based on composite electrolyte has been proposed for the first time. The stability of the fuel cell with Ni_0.6Cu_0.2Zn_0.2 composite anode has been investigated. The results indicate that the trimetallic Ni_0.6Cu_0.4−xZn_x anodes are considerable for low temperature SOFCs.     

Soybean powder enables the synthesis of Fe–N–C catalysts with high ORR activities in microbial fuel cell applications

The development of microbial fuel cells (MFCs) into a new type of carbon-neutral wastewater treatment technology requires efficient and low-cost oxygen reduction reaction catalysts in air cathodes. The use of raw soybean powder was investigated for synthesizing Fe–N–C ORR catalysts in a sacrificial SiO₂ support method. ZnCl₂ etching in the synthesis was found to facilitate the formation of hierarchical porous structures of Fe–N–C catalysts. Fe–N–C(1-1) catalyst synthesized with an optimal soybean/ZnCl₂ mass ratio of 1:1 exhibited the highest ORR activity in air cathodes. The use of the obtained Fe–N–C(1-1) catalyst enables a maximum power production of ~0.480 mW cm⁻² in MFCs, higher than commercial Pt/C (0.438 mW cm⁻²) with the same catalyst loading of 2 mg cm⁻². Long-term MFC operations demonstrated that the Fe–N–C synthesized from raw soybean have high stability and toxic tolerance, indicating that abundant low cost soybean biomass is a potential material for ORR catalyst development in MFC applications.